Field of the Application
The present application relates to a semiconductor device.
Background Art
Conventionally, a vertical type insulated gate transistor provided with a field plate is known as disclosed in Japanese Patent Application Laid-Open No. 2011-151408. Inclusion of the field plate in the vertical insulated gate transistor allows a depletion layer to be expanded in a thickness direction of a drift layer. Even when a high concentration drift layer is provided, it is possible to obtain a high withstand voltage, and thereby make a reduction of on-resistance compatible with a high withstand voltage.
Expanding the depletion layer in the thickness direction of the drift layer can reduce a feedback capacitance and an output capacitance and improve a high frequency characteristic. The feedback capacitance is a capacitance between a gate and a drain (or collector) and is a capacitance component between a gate electrode and the drift layer. The output capacitance is a sum of a capacitance between the gate and the drain (or collector) and a capacitance between the drain and a source (or emitter). In the case of a MISFET, for example, the output capacitance is defined as the sum of a gate-drain capacitance and a drain-source capacitance. In the above-described prior art, a width of the field plate in the trench is constant and an electric field of the field plate uniformly extends to the drift layer. For this reason, inclusion of the field plate considerably promotes the depletion even in a region away from the gate electrode. As a result, attempting to promote the depletion in the vicinity of the gate electrode to reduce the feedback capacitance and output capacitance would increase on-resistance, resulting in a problem that a saturation current is lowered.